1. Field of the Invention
The embodiment discussed herein is related to a switching power supply apparatus and, more particularly, to a switching power supply apparatus which includes a current resonance DC-DC switching converter and which switches an output voltage by receiving a standby signal.
2. Background of the Related Art
Because current resonance DC-DC switching converters are suitable for increasing efficiency or reducing thickness, they are widely adopted in switching power supply apparatus of television sets and the like. Some loads of switching power supply apparatus operate in normal mode at normal usage time and operate in standby mode at standby time. Such a load transmits a standby control signal to a switching power supply apparatus when it is in standby mode. The switching power supply apparatus which receives the standby control signal exercises control so as to supply to the load minimum power which the load needs to operate in the standby mode (see, for example, International Publication Pamphlet No. WO2011/065024).
FIG. 9 is a circuit diagram which illustrates an example of the structure of a conventional switching power supply apparatus. FIG. 10 illustrates an example of the structure of a control IC. In the following description the same numeral may be used for representing the name of a terminal and a voltage, a signal, or the like at the terminal.
As illustrated in FIG. 9, with a conventional switching power supply apparatus an input capacitor C1 is connected to input terminals 10p and 10n. An input DC voltage Vi is applied to the input capacitor C1. The input voltage Vi may be a DC voltage obtained by rectifying and smoothing an AC source voltage or a constant high DC voltage generated by a power factor correction circuit.
Furthermore, a circuit including a high-side switching element Q1 and a low-side switching element Q2 connected in series is connected to the input terminals 10p and 10n to make up a half bridge circuit. In this example, n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) are used as the switching elements Q1 and Q2.
A connection point common to the switching elements Q1 and Q2 is connected to one end of a primary winding P1 of a transformer T1 and the other end of the primary winding P1 is connected to a ground via a resonance capacitor C5. The magnetizing inductance of the primary winding P1 of the transformer T1, leakage inductance between the primary winding P1 and secondary windings S1 and S2, and the resonance capacitor C5 make up a resonance circuit.
One end of the secondary winding S1 of the transformer T1 is connected to an anode terminal of a diode D3. One end of the secondary winding S2 of the transformer T1 is connected to an anode terminal of a diode D4. Cathode terminals of the diodes D3 and D4 are connected to a positive electrode terminal of an output capacitor C6 and an output terminal 11p. A negative electrode terminal of the output capacitor C6 is connected to a connection point common to the secondary windings S1 and S2 and an output terminal 11n. The secondary windings S1 and S2, the diodes D3 and D4, and the output capacitor C6 make up a circuit which rectifies and smooths an AC voltage generated in the secondary windings S1 and S2 and which converts it to a DC voltage. This circuit is an output circuit of the switching power supply apparatus. The output terminals 11p and 11n are connected to a load (not illustrated).
The output terminal 11p is connected via a resistor R8 to an anode terminal of a light-emitting diode of a photocoupler PC1. A cathode terminal of the light-emitting diode is connected to a cathode terminal of a shunt regulator SR1. A resistor S6 is connected to the anode terminal and the cathode terminal of the light-emitting diode. That is to say, the resistor S6 is connected in parallel with the light-emitting diode. An anode terminal of the shunt regulator SR1 is connected to the output terminal 11n. The shunt regulator SR1 has a reference terminal connected to a connection point of resistors R9 and R10 connected in series between the positive electrode terminal and the negative electrode terminal of the output capacitor C6. A circuit made up of a resistor R7 and a capacitor C7 connected in series is connected between the reference terminal and the cathode terminal of the shunt regulator SR1. The shunt regulator SR1 causes a current corresponding to the difference between an internal reference voltage and a voltage obtained by dividing an output voltage Vo (voltage across the output capacitor C6) to flow to the light-emitting diode. A collector terminal of a phototransistor of the photocoupler PC1 is connected to a FB terminal of a control integrated circuit (IC) 12. An emitter terminal of the phototransistor is connected to the ground. A capacitor C2 is connected to the collector terminal and the emitter terminal of the phototransistor. That is to say, the capacitor C2 is connected in parallel with the phototransistor. The photocoupler PC1 and the shunt regulator SR1 make up a feedback circuit which feeds back an error between the output voltage Vo and the reference voltage to the control IC 12.
The control IC 12 has a VH terminal connected to a positive electrode terminal of the input capacitor C1 and a GND terminal connected to the ground. Furthermore, the control IC 12 has a HO terminal connected via a resistor R1 to a gate terminal of the high-side switching element Q1 and a LO terminal connected via a resistor R2 to a gate terminal of the low-side switching element Q2. In addition, the control IC 12 has a VB terminal, a VS terminal, a STB terminal, a CA terminal, an IS terminal, and a VCC terminal. A capacitor C4 is connected between the VB terminal and the VS terminal. The VS terminal is connected to the connection point common to the switching elements Q1 and Q2. One end of a capacitor Cca is connected to the CA terminal. The other end of the capacitor Cca is connected to the ground. The IS terminal is connected to a connection point common to a capacitor Cs and a resistor Rs. A circuit made up of the capacitor Cs and the resistor Rs connected in series is connected in parallel with the resonance capacitor C5. The circuit made up of the capacitor Cs and the resistor Rs connected in series is a shunt circuit which shunts a resonance current. A current shunted by the shunt circuit is converted to a voltage signal by the resistor Rs for current detection and is inputted to the IS terminal of the control IC 12 as a signal indicative of the resonance current. The VCC terminal is connected to a positive electrode terminal of a capacitor C3 and a negative electrode terminal of the capacitor C3 is connected to the ground. Furthermore, the VCC terminal is connected to an anode terminal of a diode D2 and a cathode terminal of the diode D2 is connected to the VB terminal. The VCC terminal is connected via a diode to an auxiliary winding of the transformer T1 (not illustrated for the sake of the simplicity of FIG. 9). After the switching power supply apparatus is started, the capacitor C3 is charged by a voltage generated in the auxiliary winding and is used as a power source for the control IC 12.
Furthermore, the switching power supply apparatus receives a standby control signal nml from the load (not illustrated). That is to say, one end of a resistor R11 is connected to the connection point of the resistors R9 and R10 which make up a circuit for dividing the output voltage Vo. The other end of the resistor R11 is connected to a drain terminal of a transistor Q3. A source terminal of the transistor Q3 is connected to the ground. A gate terminal of the transistor Q3 is an input terminal for the standby control signal nml. The resistor R11 and the transistor Q3 make up a mode switching circuit which receives the standby control signal nml and which switches the output voltage Vo by switching a division ratio of the output voltage Vo.
Furthermore, the output terminal 11p is connected via a resistor R12 to an anode terminal of a light-emitting diode of a photocoupler PC2. A cathode terminal of the light-emitting diode is connected to a drain terminal of a transistor Q4. A source terminal of the transistor Q4 is connected to the ground. A gate terminal of the transistor Q4 is an input terminal for the standby control signal nml. A collector terminal of a phototransistor of the photocoupler PC2 is connected to the STB terminal of the control IC 12. An emitter terminal of the phototransistor is connected to the ground. A capacitor C9 is connected to the collector terminal and the emitter terminal of the phototransistor. That is to say, the capacitor C9 is connected in parallel with the phototransistor. When the switching power supply apparatus is made to operate in normal mode, the standby control signal nml is at a high (H) level. When the switching power supply apparatus is made to operate in standby mode, the standby control signal nml is at a low (L) level. Accordingly, when the standby control signal nml is at a high level, the resistors R10 and R11 are connected in parallel. In addition, the light-emitting diode of the photocoupler PC2 emits light to turn on the phototransistor. By doing so, the STB terminal of the control IC 12 is made a ground level. Conversely, when the standby control signal nml is at a low level, the resistor R11 is excepted from a circuit. In addition, the light-emitting diode of the photocoupler PC2 goes out to turn off the phototransistor.
As illustrated in FIG. 10, the control IC 12 includes a start circuit 21 whose input terminal is connected to the VH terminal. An output terminal of the start circuit 21 is connected to the VCC terminal and a low-side drive circuit 25. An input terminal of an oscillation circuit 22 is connected to the FB terminal and an output terminal of the oscillation circuit 22 is connected to a controller 23. A high-side output terminal of the controller 23 is connected to an input terminal of a high-side drive circuit 24 and a low-side output terminal of the controller 23 is connected to an input terminal of the low-side drive circuit 25. An output terminal of the high-side drive circuit 24 is connected to the HO terminal and an output terminal of the low-side drive circuit 25 is connected to the LO terminal. The high-side drive circuit 24 is also connected to the VB terminal for a high-side power source and the VS terminal which is a high-side reference potential. The CA terminal and the IS terminal are connected to a load detection circuit 26. The load detection circuit 26 is connected to the controller 23. Furthermore, the control IC 12 includes a standby signal generation circuit 27 whose input terminal is connected to the STB terminal. An output terminal of the standby signal generation circuit 27 is connected to the controller 23. A standby signal sdymo generated by the standby signal generation circuit 27 is supplied to the controller 23.
With the switching power supply apparatus having the above structure, before the control IC 12 begins switching control, the start circuit 21 of the control IC 12 which receives the input voltage Vi at the VH terminal supplies a starting current to the capacitor C3 to charge the capacitor C3. After the capacitor C3 is charged, its voltage VCC is supplied to the VCC terminal and the low-side drive circuit 25. After the control IC 12 begins switching control, the voltage VCC is supplied from the auxiliary winding of the transformer T1.
It is assumed that the standby control signal nml supplied from the load is at a high level indicative of the normal mode. Then the transistor Q3 is turned on. As a result, the resistors R10 and R11 are connected in parallel. The transistor Q4 is also turned on and the STB terminal of the control IC 12 is made a ground level by the photocoupler PC2. Because the STB terminal is made the ground level, the standby signal generation circuit 27 supplies a low-level standby signal sdymo to the controller 23.
The control IC 12 exercises control so as to alternately turn on and off the switching elements Q1 and Q2. By doing so, the output voltage Vo of the output circuit on the side of the secondary windings S1 and S2 of the transformer T1 is kept at a determined value. That is to say, the shunt regulator SR1 detects the output voltage Vo by the use of an output of a voltage division circuit made up of the resistors R9 and a circuit made up of the resistors R10 and R11 connected in parallel, and outputs a current corresponding to an error between a detected value and the determined value (internal reference voltage of the shunt regulator SR1). The error current is fed back to the FB terminal of the control IC 12 by the photocoupler PC1. In the control IC 12, the oscillation circuit 22 adjusts an oscillation frequency according to a signal FB at the FB terminal and the controller 23 supplies to the high-side drive circuit 24 and the low-side drive circuit 25 a signal which makes the switching elements Q1 and Q2 turn on and off alternately. By doing so, the switching elements Q1 and Q2 are on-off controlled. As a result, the amount of power induced on the secondary side of the transformer T1 is adjusted and the output voltage Vo is kept at the determined value.
Next, when the standby control signal nml supplied from the load becomes a low level indicative of the standby mode, the transistors Q3 and Q4 are turned off. As a result, the STB terminal of the control IC 12 becomes a high level. Accordingly, the standby signal generation circuit 27 outputs a high-level standby signal sdymo and the control IC 12 is switched from the normal mode to the standby mode. The output voltage Vo outputted at the time of the control IC 12 being in the standby mode is switched to a voltage lower than a voltage outputted at the time of the control IC 12 being in the normal mode. This output voltage Vo is detected by the shunt regulator SR1 by the use of an output of a voltage division circuit in which the resistor R11 is excepted, and an error voltage is fed back to the control IC 12. By doing so, the output voltage Vo is kept at a determined value smaller than the determined value at the time of the control IC 12 being in the normal mode. By decreasing the output voltage Vo at the time of the control IC 12 being in the standby mode, the efficiency of the switching power supply apparatus is improved and the standby power consumption of a system including the load is reduced.
With the above switching power supply apparatus a signal transmission circuit including an element which performs transmission in an electrically insulated state is needed for transmitting from the secondary side to a control IC on the primary side a standby control signal supplied from a load. The control IC needs a specialized terminal for receiving the standby control signal. This raises the costs.